Either you have:
a) Bad disk.... Scratched, dirty e.t.c
b) Faulty DVD Rom Drive that is giving read errors
b.1) Faulty Memory/ Hard Drive
c) Illegal/Pirated version that has been mucked around with and is now corrupt
Solutions:
a) Clean disk or buy new
b) Buy new drive... £12.00 from ebuyer.co.uk
b.1) Buy new memory/Hard drive
c) Buy the disk
Hope this helps